It is well recognized by persons skilled in the art of oil recovery techniques that only a fraction of the amount of petroleum or oil initially present in the petroleum reservoir is recovered by primary recovery methods, e.g. by pumping or allowing the oil to flow to the surface of the earth as a consequence of naturally-occuring energy forces, or even when secondary recovery processes such as waterflooding are employed. Although waterflooding is an inexpensive and commercially successful enhanced oil recovery process, water does not displace oil efficiently even in those portions of the formation through which it passes, because water and oil are immiscible and the interfacial tension between water and oil is quite high. This reason for the failure of waterflooding to recover all or even a substantial portion of the oil remaining in the formation after completion of primary recovery operations has also been recognized by persons skilled in the art of enhanced oil recovery, and many literature references suggest the incorporation of a variety of surface active agents (e.g., surfactants) in the flood water, for the purpose of reducing the interfacial tension between the injected aqueous fluid and the formation petroleum, which accomplishes an increase in the amount of oil displaced by the injected fluid. Petroleum sulfonates have been suggested in many references and have been employed in field trials of surfactant waterflooding oil recovery process, with varying degrees of success, although petroleum sulfonates can only be used without additional solubilizing surfactants in formations containing relatively low salinity water, e.g. formations containing water whose salinity is less than about 20,000 parts per million total dissolved solids.
It is also well recognized that other types of surfactants may be used alone or in combination with petroleum sulfonates in high salinity water-containing formations. Alkylpolyethoxy sulfates or alkylarylpolyethoxy sulfates may be used alone or as a solubilizing co-surfactant for petroleum sulfonate in high salinity water-containing formations so long as the formation temperature is less than about 140.degree. F. (60.degree. C.). Ethoxy sulfates are not recommended for use in higher temperature formations because of their tendency to hydrolyze, and the rate of hydrolysis increases with increased temperature. It is also known in the art to employ, either alone or in combination with petroleum sulfonates, alkylpolyethoxyalkylene sulfonates or alkylarylpolyethoxyalkylene sulfonates, which are very tolerant of high salinity of water and stable at elevated temperatures.
Despite the encouraging results which have been reported in the literature employing the above-described combination processes in high salinity water-containing oil formations, the actual experience to date in field trials employing these processes have been disappointing for a variety of reasons. Adsorption of the salinity-tolerant surfactants is a serious problem, which increases the cost of the fluid and decreases the effectiveness of the oil recovery process. The amount of oil recovered in many instances has been substantially less than originally expected, based on laboratory experiments, and this is at least in part related to the above-described phenomena of surfactant loss from the fluid to the formation. Another serious problem which has been encountered at times, is an adverse interaction between the ethoxy sulfates or ethoxy sulfonates and certain hydrophilic polymeric materials such as polyacrylamide or polysaccharides commonly injected subsequent to the injection of the surfactant fluid, for the purpose of insuring a favorable mobility ratio between the injected fluid and the displaced fluids. Moreover, optimum hydrophilic polymer performance is achieved in relatively low salinity environments, but contact between low salinity polymer fluids and high salinity surfactant fluids degrades the effectiveness of a surfactant combination which is tailored to produce optimum results in high salinity environments, and so the interaction between the high salinity surfactant fluid and the low salinity polymer fluid causes some degradation of both polymer effectiveness and surfactant effectiveness.
In view of the foregoing discussion, and the current serious petroleum shortage which could be significantly alleviated with an effective and economical enhanced oil recovery process, it can be appreciated that there is a significant commercial need for an economical surfactant waterflooding enhanced oil recovery process suitable for use in high salinity environments, which avoid at least certain of the aforementioned disadvantages encountered using surfactant waterflooding processes as are now practiced.